Polyoxymethylenes synthesized by prior art techniques are classified broadly into homopolymers and copolymers. Polyoxymethylene homopolymers and polyoxymethylene copolymers of the prior art respectively have only one of the two desirable characteristics, namely either excellent mechanical property or excellent stability. That is, polyoxymethylene homopolymers obtained by homopolymerization of formaldehyde or trioxane are excellent in mechanical properties but are not satisfactory in stability; on the other hand, polyoxymethylene copolymers obtained by copolymerization of formaldehyde or trioxane with a cyclic ether or cyclic formal are excellent in stability but are poor in mechanical properties. With respect to long term resistance to heat or resistance to hot water, furthermore, even polyoxymethylene copolymers are not satisfactory.
Polyoxymethylene copolymers are usually obtained by copolymerizing formaldehyde or trioxane with a cyclic ether. U.S. Pat. No. 3,027,352 describes a copolymer obtained by cationically copolymerizing trioxane with ethylene oxide or 1,4-dioxolane.
Further, U.S. Pat. No. 3,337,503 discloses the use of methylal as a molecular weight controlling agent in the polymerization of trioxane. When methylal is used as the molecular weight controlling agent, the terminal group of the resulting polymer becomes the methoxy group.
In the polymerization of formaldehyde or trioxane or the copolymerization thereof with a comonomer using cationic catalyst, hydride shift (i.e., hydrogen abstraction) takes place, which results in scission of the main chain of the polymer and makes it impossible to obtain a high molecular weight polymer. Moreover, it is known that the terminals formed by the scission become the methoxy group and the formate group.
Further, when compounds having hydroxyl groups, such as water, methanol, formic acid, etc., are present in polymerization, they function as a chain transfer agent and form unstable terminal structures. A usually adopted remedial method is to decompose these unstable structures and stabilize the terminal groups as hydroxyethoxy groups, by application of post-treatment after polymerization. In the case of formic acid, however, terminal formate groups are formed simultaneously.
As described above, in the previous polyoxymethylene copolymers, terminal formate groups are formed during polymerization by chain transfer reaction caused by formic acid, which is an impurity in trioxane, and by hydride shift reaction, which is a side reaction caused by cationic catalyst, so that it has been difficult to reduce terminal formate groups.
The present invention relates to a polyoxymethylene copolymer which has both excellent stability and excellent mechanical property. In more particular, the invention relates to a polyoxymethylene copolymer which is excellent in stability and mechanical property and further in long term resistance to heat and resistance to hot water, and to the process for producing the same.